The present invention relates to a method for forming a fine pattern by using a patterned resist layer and a process to use the patterned resist layer in manufacturing semiconductor devices.
Heretofore, for producing fine structures such as large scale integrated circuits (LSI), a so-called photolithography using light has been employed predominantly. However, in the production of fine structures of several hundred .ANG. even if far infrared rays are used, they are too coarse by a factor of ten and it is difficult even by the use of X-rays. That is, for producing a fine structure, of several hundreds .ANG., a resolution less than that is necessary and, accordingly, it is necessary that the wavelength is less than several hundreds .ANG.. This is the region of X-rays and there are various problems to be solved at present in view of both the X-ray source and the mask material.
On the other hand, patterning by using electron beams has begun to be used recently as a substituent method. FIG. 1 and FIG. 2 show examples thereof.
FIG. 1 shows a case using a positive type resist, in which a positive type resist 2 is coated on a substrate 1 (refer to FIG. 1A), and then patterning is applied for the resist 2 by the electron beam exposure 5 and development (refer to FIG. 1B) Then, a pattern forming material 3 is deposited to form on the entire surface (refer to FIG. 1C), and then the resist 2 is lifted-off together with the pattern-forming material 3 thereover to form the pattern-forming material 3 in a predetermined pattern on the substrate 1 (refer to FIG. 1D).
FIG. 2 shows a case of using a negative type resist, in which a pattern forming material 3 is formed on a substrate 1 and a negative type resist 4 is deposited to form further thereover (refer to FIG. 2A) and patterning is applied for the resist 4 by electron beam exposure 5 and development (refer to FIG. 2B). Then, the pattern-forming material 3 is selectively etched by using the resist 4 as a mask to apply patterning for the pattern-forming material 3 (refer to FIG. 2C). Then, the resist 4 is removed (refer to FIG. 2D).
In the patterning method using the electron beams as described above, it is adapted for both of the positive and negative resists that a desired pattern is obtained by previously coating the resist to several thousands .ANG. and then changing the resist material by the electron beam exposure. The starting material for forming the pattern which is the final object in this case is a solid resist, which acts as multiple scattering material to incident electron beams. In this case, as shown, for example, by Wilkinson, et al. (Superlattices and Microstructures, Vol. 2, No. 6, 1986), traces of electrons as shown in FIG. 3A and FIG. 3B are expected. FIG. 3A shows a trace for scattered 100 electrons after applying electron beam 13 at 50 KV to a resist layer 12 of 0.3 .mu.m thickness formed on an Si substrate 11 of 5 .mu.m thickness, while FIG. 3B shows the trace of electrons after applying electron beam 13 at 10 KV to the same specimen.
As can be seen from the figure, due to the multiple scattering of electrons in the resist, their initial spacial resolution is lost during passage through the resist. It is particularly fatal in the negative type resist. In addition, due to the backward scattering electrons from the substrate, the electrons also strike the resist in other places. The drawing shows an Si substrate and the effect is increased in GaAs substrates, etc. Accordingly, since the portion through which the electrons have past is exposed inevitably, all of them hinder the production of microstructures.
As a method of forming a pattern, it has been known to dispose a substrate in a gas atmosphere containing constituent materials such as either a metal or a semiconductor material and to use irradiating electron beams at a desired portion on the surface of the substrate, thereby depositing metal or semiconductor material in a predetermined pattern on the substrate (refer to Japanese Patent Laid Open Publication No. Sho 62-42417). It has also been known to dispose a substrate in a gaseous resist atmosphere and to direct irradiating electron beams at the surface of the substrate to thereby deposit a resist of a desired pattern (refer to Applied Physics Letters. Vol. 29, No. 9, 1978; p 596-598).
Though the deposition technique of resist shown in the above literature is generally known, there is no recognition how to utilize the technique in manufacturing the semiconductor devices, because there is no suitable technique to remove the resist and trim the shape of the resist.
Further in the conventional photolithography as described above, it is possible to completely cover a pattern 22 of a trapezoidal cross section formed on a semiconductor substrate 21 with a resist 23 as shown in FIG. 4A. However, as shown in FIG. 4B, in the case of a pattern 22 of an inverted trapezoidal cross section, in which shadows are present when viewed from the incident direction of the exposure rays, the resist 23 can be formed only on the upper surface thereof and it is difficult to cover the entire surface of the pattern 22 with the resist 23. Accordingly, when isotropic etching is applied subsequently, the pattern 22 is etched from the portion not covered with the resist 23 and, as a result, the initially patterned shape can not be kept. This problem is not restricted only to the case of the pattern 22 of the inverted trapezoidal section as shown in FIG. 4B, but it is always brought about generally in the case of patterns having portions forming shadows when viewed from one direction.